The purpose of the study is to characterize the pharmacokinetics and pharmacodynamics of Procysteine (L-2-oxothiazolidine-4-carboxylic acid (OZT), a cysteine prodrug), in patients with amyotrophic lateral sclerosis (ALS). The pharmacokinetic data obtained in this study will be used to design a future phase III study of Procysteine in patients with ALS.